This invention relates to telecommunication cables.
The specified tensile strength and operating temperature ranges of certain fiber optic cables are based upon the expected limits for longitudinal thermal expansion and contraction and the physical compression and tension to be experienced by the cable. The object is the avoidance of tension on or excessive bending of the optical fibers, which leads to attenuation of the transmitted signal or damage to the optical fibers. The cables also have a specified minimum bend radius. If the cables are bent more sharply than allowed by the minimum bend radius during installation, kinking may result, causing damage to the optical fibers, also known as light waveguides.
Thermoplastic polyolefin materials have many characteristics desirable for use in cables, as these materials are chemically resistant, hydrolytically stable, not environmentally harmful, easily processed, and low in cost. High coefficients of thermal expansion and high levels of shrinkback, defined below, are their principal limitations. Excessive shrinkage of tubes made of thermoplastic materials which are components of communications cables can lead to excessive compression on or excessive bending of the communication elements in those cables. Excessive thermal expansion of such tubes can lead to excessive tension being applied to the communication elements. Shrinkage and expansion of cable components made of these plastic materials are major factors to be considered in determining the operating temperature range of the cable.
Exact tube shrinkage control is critical in the extrusion of cable buffer tubes containing optical fibers. The amount of excess optical fiber length as compared to the length of the buffer tube substantially affects the amount of bending of the optical fibers at low temperatures to be experienced by the cables. Cables are sometimes designed to have an anticipated excess fiber length. Excessive tube shrinkage increases the excess fiber length, and may cause a cable to fail to perform according to its specification.
Control of shrinkage and expansion of cable outer jackets and central member jackets are also important because they typically contain a relatively large volume of plastic material. Therefore, relatively large forces can be exerted in the longitudinal direction on the other cable components.
It is known that cable components made of plastic materials shrink when changing from the liquid to the solid phase during processing. Plastics made of crystalline materials exhibit such shrinkage significantly more than plastics made of non-crystalline amorphous materials. Some contraction occurs immediately upon solidification. Further contraction occurs as the plastic cools to room temperature. Secondary crystallization occurs over longer periods of time thereafter, causing shrinkage that is sometimes referred to as "shrinkback", in the case of uncabled buffer tubes holding one or more optical fibers. Shrinkback is not a significant factor for jackets because other cable components inhibit such shrinkage, except possibly at the cable ends. Hereinafter the expression "post-extrusion shrinkage" is used to designate the combination of a portion of the contraction due to cooling of a solidified buffer tube holding an optical fiber, and the shrinkback that occurs within the subsequent seven days. The portion of the contraction due to cooling that is included in the term "post-extrusion shrinkage" is the contraction that occurs beyond that point in the extrusion line where the optical fibers and the buffer tube become coupled. Defined in this way, the post-extrusion shrinkage is numerically equal to the so-called excess fiber length in the buffer tube seven days after extrusion.
Environmentally induced thermal contraction and expansion as well as shrinkback in tubes has been controlled by coupling tubes to adjacent cable components that do not experience shrinkage, by the addition of non-crystalline materials to the extrudate, or by the addition of rigid filler materials, typically inorganic, that remain in the solid state during cable processing. The filler materials usually have surface treatments allowing them to couple sufficiently to the plastic materials, and the filler materials also must have sufficient compressive strength to control shrinkage of the tubes made of the plastic materials incorporating the filler materials.
In particular, embedding longitudinally oriented reinforcement fibers in a tube lowers the expansion or contraction of the tube during temperature changes because the plastic of the tube generally has a higher thermal coefficient of expansion than the reinforcement fibers. For a buffer tube, shrinkback is also reduced.
One disadvantage of the use of the above-mentioned filler materials is cost. Another is that their stiffness, though necessary for the filler materials to perform their intended function, causes the cable elements incorporating the filler materials to have too little flexibility. The composite materials have too low an elongation to break, causing them to be too brittle. In addition, the cable elements have an undesirably large diameter at kink.
U.S. Pat. No. 5,307,843 discloses an apparatus and method for extrusion of thermoplastic tubes reinforced by fibers of 1 cm in length of Kevlar, glass, carbon, boron or ceramics in a proportion of 5 to 40% by weight, particularly 20-30% by weight. The stated objects are to reduce the risk of rupture of a tube carrying a fluid due to longitudinal cracks and to improve the tube's surface smoothness. The process is stated to enable fibers to be directed perpendicularly to the axis of the tube. The problem of tube longitudinal dimensional stability is not addressed, as exact tube length is not critical for tubes carrying fluids.
U.S. Pat. No. 4,693,551 discloses polyethylene buffer tubes having embedded therein a plurality of glass fibers substantially oriented in the longitudinal direction along the tube. The stated object is to contribute to cable tensile strength. The reinforcement fibers have a length of about 0.5 cm. The volume of glass fibers in the tube material is up to 30%, preferably 25 to 30% by volume, estimated to equal 45% to 52% by weight. It is believed that such buffer tubes would be relatively stiff, and the problem of tube longitudinal dimensional stability is not addressed.
U.S. Pat. No. 4,956,039 discloses a core strength member coated by polypropylene filled with short E-glass fibers which are chemically coupled to the polypropylene. The exact length of the fibers is unstated. The degree of filling of the glass fibers is from 10% to 30%, a preferred value being 20%. However, a second layer of thermoplastic is also applied over the filled polypropylene layer to stiffen the end structure, to reduce the danger of kinking.